ps. Some really cool guys from a Greek forum helped me in the past to solve this problem (by "breaking" the parallel side walls with some wooden constructions) but i need to try something different at this time.
Depending on how much of a slope you have I think you would be better not to sit in the middle of the room. It is always better to slope from front to back, but you can also put a cloud there to take away the reflection.
Flutter can be a bitch. One thing you could do is play a clap through your monitors, sit in your mix spot and listen to see if you really can hear all the flutter. Sometimes people walk around the room chasing it (or sit in there mix spot and clap) when really it is important what you can only hear in the mix spot coming from the monitors.